It has been recognized that the T1/E1 rate (1.544/2.048 Mbit/s) is a cost effective way of user access to an ATM network as well as connection between ATM network switches. However, as ATM technology for wide area networks is deployed more and more, demands for transmission links of a rate higher than T1/E1 are increasing. Links of higher rates, such as T3/E3 (44.736/34.368 Mbit/s), have been designed to meet these needs. However, the cost of T3/E3 links is still prohibitive in many cases and the ratio of cost versus realistic utilization of the entire rate is not always attractive and fully justified for new ATM end users and service providers. ATM inverse multiplexers (AIMs) have been proposed to satisfy the need by using multiple T1/E1 links which are grouped collectively to provide the service at a higher rate.
FIG. 1 and FIG. 2 show two sample configurations in which AIMs are used. FIG. 1 depicts a user access to a network through user network interfaces (UNIs) and FIG. 2 a link connection between ATM switches through broadband inter-carrier interfaces (BICIs) or private network to network interfaces (PNNIs).
Referring to the figures, the basic function of AIMs is to work in pairs to take an ATM cell stream coming from the ATM layer, send it over the multiple links by spreading cells over the available links and ensure that the initial cell stream can be retrieved at the far end. Thus the AIMs preferably make the ATM traffic transparent to the ATM layer over multiple links which connect them. As far as the ATM layer is concerned, it should only see a pipe whose rate is now the sum of the multiple link rates. It is assumed that each link is run in clear-mode without the presence of intermediate ATM nodes processing ATM cells. This means that there should be no cell discard by any intermediate transmission equipment.
Currently no ATM inverse multiplexing protocols have been proposed which can properly interwork existing ATM inverse multiplexers or other ATM products which are already available on the market, and yet are flexible enough to fit into the current standard ATM specifications. Two proposals for an ATM inverse multiplexing protocol have so far been made and are described in detail below.
New Transmission Convergence Protocol Using GFC Bits
This protocol was presented in "Physical Layer Sub-Working Group ATM Forum/94-0775, ATM Inverse Multiplexing Mechanism", September 1994, by StrataCom Inc. The protocol robs two of the Generic Flow Control (GFC) bits contained in each cell transmitted over the multiple T1/E1 links to implement a new transmission convergence (TC) layer. FIG. 3 shows the ATM cell structure which is defined in the ITU Recommendation. The TC layer is defined by one GFC bit for framing and the other one for link control. The framing bit is used to determine relative link delays while the link control bit is used for communication, control and administration between two TC points at two ends of the inverse multiplexer.
In order to establish the sequence of cells over the links in a round robin manner, one end is defined as being "master" and the other as "slave". The "master" decides and informs the slave about the multiple link configuration using the control channel implemented through the link control bits.
This protocol is only applicable, however, for UNI application points because GFC bits that are robbed to implement the TC layer are only present in a cell defined for UNI. For NNI cells, the corresponding bits are no longer available since they are captured under the VPI field. Service providers are interested in ATM inverse multiplexers for carrying ATM traffic at rates higher than T1/E1 and lower than T3/E3, but this protocol will not satisfy their need. It should also be noted that the protocol calls for a need to identify a "master" and a "slave" TC point and that requires an additional setting to be performed by the network operator.
Bit Pipe Inverse Multiplexing
This protocol was presented in "Physical Layer Sub-Working Group ATM Forum/94-0956, Inverse Multiplexing of ATM cells over low speed UNIs such as T1 and E1", September 1994, by Digital Link Corporation. It proposes a "bit pipe" inverse multiplexing technique requiring the definition of a "bonding" (bandwidth on demand) like specification for N (positive number) T1/E1 inverse multiplexing.
It is not clear in the proposal how both ends of the links exchange information concerning the order of cells to be transferred from one end to another over multiple links. The proposal mentions the existence and deployment of physical layer protocols that perform inverse multiplexing. The inverse multiplexer which can be used in this proposal is presumably the one defined by Digital Link Corporation in their "DL3800 DS1 Inverse Multiplexer Users Manual, 1993".
The inverse multiplexing protocol defined in the above user's manual relies on the definition of an extra bit taken from T1/E1 payload bits to configure the multiple links and adjust differential link delays. This protocol introduces the need for extra processing of data between devices dealing with T1/E1 frames and ATM cell delineation. It also causes the ATM cells to no longer be byte aligned with the DS1/E1 frame. This is a requirement by the ATM Forum UNI DS1/E1 Physical Layer specifications. Changes like this would not be welcome by end users, vendors and service providers who are already using and deploying ATM equipment.
U.S. Pat. No. 5,608,733, Mar. 4, 1997, Vallee et al, describes good ways of obviating the above noted problems. The patent uses ATM sequence number cells indicating a specific round robin order of a plurality of transmission links over which ATM data cells are transmitted. The ATM sequence number cells also indicate whether or not a destination is ready to receive ATM data cells in that specific round robin order.
The present invention extends further variety of functionalities which are useful in inverse multiplexing.